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    • CommentTimeFeb 22nd 2008

    Just a sanity check here from anyone who knows part L1B well please. I'm 99 % certain my understanding is correct.

    I'm planning a small extension (small floor area but with big expensive glass).

    There are 3 potential levels for the calculations:
    1)Basic 25% rule of floor area with allowance for windows and doors removed (no good for me, not enough glazing allowed)
    2) Comparison of area weighted U value of all the extension elements as planned, with area weighted U value of a "theoretical" extension meeting the tabulated minumum U value standads and "opening area" of paragraph 15.
    3) Full house SAP offset calculations.

    Method 2 seems to be the way to go for me, if my understanding is correct, only it seems a little too "easy". I am making my "theoretical" reference extension based on 25% of the new floor area PLUS the area of windows/doors I will remove (this added area is significant because I'm taking out a 1970's single glazed front door and glass surround). This means that by using some reasonably decent glass (overall U < 1.4 say), and a good ceiling and floor, it's easy to have a really big glass area in the extension.

    My question is then. Is it correct to assume that my "theoretical" extension I use for comparison with the real extension, is allowed to have glass+doors according to:
    (a) 25% PLUS removed door/window area .
    (b) 25% only, with no allowance for removed door/window area.

    According to my understanding of paragraph 18 in L1B, I'm assuming (a) is the case, but have my slight doubts that building regulations controllers see method (b) as correct.
    • CommentAuthorJoinerbird
    • CommentTimeFeb 22nd 2008
    Erm .....b) I think.

    Im not telling you where I work, but with a small extension, I would ask for better Uvalue floor or wall or roof say, to allow you to have your big windows. And / or I would perhaps ask your to up grade you loft insulation to current standards to off set your big windows.
    • CommentAuthorHelen
    • CommentTimeFeb 22nd 2008
    I'd go for a) myself, Paragraph 15 clearly says 'the sum of' 25% floor area plus the existing windows to be covered up and paragraph 18 refers you straight back to that so I don't think theres much room to manoeuvre there.

    If you need to make a point like this cite and quote the regulations you're relying on in your B.Regs application, and if it gets turned down ask for similar in return. Do however do it extremely politely and don't get stroppy if the answer is 'no', a suprising amount of Building Control is left to the discretion of the officer and you may find yourself having a hard time with regs you didn't even know existed. Be careful not to win the battle and lose the war.

    I'm not saying where I work either, but the phrase 'gamekeeper turned poacher' has been used!:smile:
    Hi Justin.

    I agree with the a) option. I have obtained many approvals on this basis, but don't forget that the U-values for the "covered over" openings added to the 25% of floor area are fixed in the regs. Having better than minimum U-values for the new extension can sometimes help.
    I have pruduced a spreadsheet for calculating and printing out to send to building control that helps me play with the figures. You are welcome to have a copy of if you can send me an E-mail address. Mine is : charlieduke@tiscali.co.uk

    The full SAP 2005 calc can be used if you are carrying out improvements to the main house at the same time, ie adding insulation or updating the boiler. I recently gained approval for a client's glazed kitchen extension because the old 65% eff boiler was being changed for a 92% one, saving massive carbon emmissions even with the high loss extension.

    Good luck.
    I used a) last summer when doing a similar extension. In our case we were using an existing slab with no insulation under it. I didn't compare the two extensions through. I compared the house as it was before with a theoretical extension built to regs, with the house as it was after (with the extenson I actually built). Doing it this way, I was able to offset the lower U-value of the uninsuated floor (and a fair bit of glazing) with an increase from 100mm to 270mm of the loft insulation in the whole house. It passed (and was cheap... as the occupier is a pensioner and got the loft insulation done free!).
    • CommentTimeFeb 23rd 2008
    Thanks for the replies. I see from your comments (Thanks Joinerbird and Helen) that it's perhaps not always handled the same way by different.. er.. inspectors. I think I'm encouraged overall that I can use (a).. which might just see me through if I spend enough on the glass!

    Just to answer some other points.
    Joinerbird: Loft insulation might normally be a useful offset if I were to go for the whole house SAP, only I have already (4 years ago) increased my loft insulation to about 400mm, so it might be pretty hard to claim this as a credible offset against my planned extension! I have also put in a woodburner but I did this also 18 months ago, and again it might be hard to use this for any whole house offset SAP calc as an offset against the starting point. Having done this, the next economic method if I were to use the whole house SAP offset would be to improve internal wall and ceiling insulation round my integral garage (single brick!), and after that by replacing existing DG windows with better ones. I'd like to and keep the extension as a self-contained unit for method (2). (I have actually got quite a long way with SAP form already, but would like to keep this only as a backup if I can't make the numbers work by the second method).

    Charlie: yes, my U vales for the "real" extension are (where I can do it) going to be better than the part L1b requirements, whilst my "theoretical" extension uses the tabulated figures (Including the dreadful yet useful U=3 for a front door!!). This all helps the cause. Regarding the spreadsheet, thanks, I have been doing my own spreadsheet for these calcs, but if you have a proven format which has been accepted, then I might find something from that useful. I'll drop you an email. Rgds.

    Chris; thanks, yes. As I mentioned explained. I would have had ample scope for offsetting the extension using (almost ineffective) previous loft insulation with 400mm I now have. Yes it costs very little, Only doing this 4 years ago may not hold water as an effective offset for a futeur extension.

    So Helen and Joinerbird: Would I have any credibility trying to offset existing old work against my extension in SAP? I am not seriously thinking of trying this one on, (though I might if given a hint it could work). I'd sooner contain the job on it's own, and hence my reliance on tight calculations for the extension.

    • CommentAuthorEd Davies
    • CommentTimeFeb 23rd 2008
    "Loft insulation might normally be a useful offset if I were to go for the whole house SAP, only I have already (4 years ago) increased my loft insulation to about 400mm, so it might be pretty hard to claim this as a credible offset against my planned extension!"

    How government rules backfire. Some friends of mine are putting up an extension (actually, replacing an existing rickety wooden hut-like thing which is sort of an extension). I can't remember the exact details but to help with the insulation requirements they've put off replacing the single glazing in the main part of the house with double glazing until the extension project starts; rules meant to save energy have resulted in them burning more oil for one or two winters. Humph.
    • CommentAuthortony
    • CommentTimeFeb 23rd 2008
    Why do want so much glass in it any way aren't you going to be trying to heat the gardens?
    • CommentTimeFeb 23rd 2008
    I perform Part L1B compliance check and most have more then 25% glazing

    Going down the full SAP route is to be avoided as building controls are generally not happy with this approach; The view that has been expressed to me is that the extension should be of sufficiently high build quality to stand on its own merits.

    I actualy agree with this - in term of heat loss/gain and thermal comfort the extension may be performing badly. The exception to this would be when the space is designed to integrate with the whole house functioning as, for example, a sun space.

    Following the SAP route has big issues. Most significant being the assessment of thermal bridging and air leakage. As SAP heat loss is so dependent on ventilation a can of worms is opened... The process has not been clearly thought through by the Part L committee. (SAP includes an allowance for curtains! So windows with U values of 2.0 drop to 1.85 straight away, yet you are expected to survey and analyse the whole building to achieve this improvement!).

    So, to follow the Area weighted route...

    Firstly a Notional extension is calculated with a 25% opening to floor ratio + the are of any opening now incorporated into the extension.
    Notional U values are used.

    This is then compared to as built.

    The Heat Loss Coefficient (HLC) / Area weighted U value should be the same or an improvement compared to the Notional design.

    From experience I have found that the heat loss though the glazing will dominate - You can expect well over 50%.

    Whilst the walls, floors and roof should all, reasonably, be U<=0.2 this is not goung to help much.

    The key is that the notional glazing has a U value of 2.2! This is very generous and means that even just argon filled standard pvc glazing will allow you a ratio of 30%

    Good glazing at, say U=1.3 allows you 42%.

    Throw in some old openings and 50% glazing ratio is easy to achieve.


    Existing: 4m2 of door and windows

    Extension - 2 x 5 with flat roof , 2.4 high.
    FLoor are 10m2
    Wall area 21.6m2
    Notional HLC - 25.99

    Lets make this a conservatory with good floor insulation, U = 0.15 (not hard to do)

    To pass, use triple glazing with U = 0.75

    So you can still have a heated conservatory, at a price.

    (for just glazed walls floor and foor would be U = .1 and glazing U=1.1, just possible with very high spec double glazed insulated frames)
    • CommentTimeFeb 25th 2008
    Hi PaulT.

    Thanks, Yes these are similar numbers to ones I'm working with. I'll continue to work on keeping SAP out of it then if I possibly can. I agree that it seems a little contrived to use the pretty poor minimum standards in L1b as a tool to make the real extension look good. - From a practical point of view i'll continue to improve the rest of the place wherever I can. I also hope to gain some winter solar gain from the glass.

    For Tony: "...aren't you going to be trying to heat the gardens? ". Good point. I don't want to do that, and am striving for the right compromise. Have a think about the picture (attachment), and picture this though.. This is the only side of my house with a proper garden, it faces South, it's elevated and it has unobstructed views over 80 Ha of rolling deer parkland, it has no road on this side, the view is about 20 Miles, it's my garden, there are no windows.

    Blame 1970's builders, but we bought the place almost entirely for the location and would like to try and do something. Hence some glass!

    Suggestions welcome, but it's a major overhaul to upgrade the flat roof kitchen, removal of porch and 1970's internal door+glass. Replacement with and about 24M^2 added floor area/kitchen in SE corner, with up to 15M^2 of combined glass+door area. Juggling with the aesthetics versus the glass area versus my concience. It looks just about do-able with L1b, but I need to use offsets allowed for as we've been discussing, good insulation and some good glass. Winter solar gain should be helpful, but that wouldn't help for anything as far as regulations go unless I go the whole hog with SAP.

    • CommentAuthorJoinerbird
    • CommentTimeFeb 26th 2008
    Gosh Justin, Im sooooo sorry, I re-read my post and.....I meant A!!!!

    I wouldnt tell my BCO I already had 400mm in the loft......(tut tut)
    If the views are so good, how about a flat roofed extension with roof garden accessed through a new a door from the upstairs? I'd extend the flat roof a couple of feet to create an overhang above the south facing windows else you'll roast in summer.
    • CommentTimeFeb 26th 2008
    Thanks Chris. Agreed, I was considering some sort of timber brise soleil.
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